Les risques conchylicoles en Baie de Quiberon. Troisième partie : le risque d’hypoxie pour l’huître creuse Crassostrea gigas. Rapport final du projet Risco 2010-2013

The project “Risco”, supported by the “Pôle Mer” and funded by the Regional Council of Brittany, deals with specific risks of mortality of oysters, Crassostrea gigas, cultivated on the bottom, in a subtidal bay of South Brittany : the bay of Quiberon (56, France). Massive mortalities of oysters were reported in summer 2006 in this bay, exclusively located in the deep muddy area with a positive gradient eastward. A validated biogeochemical model was applied in order to simulate the dissolved oxygen over 2000-2006 : it revealed several episodes of hypoxia, more or less intense according to years, but with the same spatial distribution. This approach proved 2006 to be the most hypoxic year since 2000. The hypoxia was due to a rare conjunction of 3 factors : (a) a local upwelling generated by north-west winds, during a neap tide; (b) an abnormal high temperature of coastal waters; (c) probably an intense phytoplankton bloom in summertime. Due to the stratification induced, oxygen consumption near the bottom exceeded its renewal. The hydrodynamism of Mor Bras, at a larger scale, excludes any import of hypoxic water from the nearby “Baie de Vilaine”, whatever the wind or tide regime. The simulated hypoxia area fitted fairly well to the 2006 mortalities. In 2010, experimental oysters deployed at 15 stations and monitored monthly, exhibited also a lower growth rate in the same area, in spite of higher chlorophyll concentration. The application of a Dynamic Energy Budget (DEB) model to growth data confirmed the responsibility of hypoxia in abnormally slow growth rates. So hypoxia may be considered as a stressful factor limiting growth prior to mortalities. It may be concluded from our study that hydro-climatic and trophic conditions have the capacity to deplete oxygen in bottom coastal zones with possible consequences on biotopes and cultivated species: farm yields may be severely affected. This study will allow to manage more closely the commercial risk of shellfish farming at spatial and temporal scale.